1. Field of the Invention
The present invention generally concerns automatic gain control of sound amplifier circuits. More particularly, the present invention concerns an automatic gain control circuit for use with a hearing aid device or other device used for speech detection and amplification.
2. Background Information
Circuits used for speech detection, and for aiding the hearing of the partially deaf and hearing impaired, are well known. Many of these circuits incorporate automatic gain control (AGC). One problem which must be dealt with by circuits augmenting and aiding human hearing is that a person with impaired hearing likes to hear the normal environmental sounds around him, but these same sounds interfere with speech discrimination and need to be suppressed in the presence of speech. Simultaneously, a person with impaired hearing usually is sensitive to a limited amplitude dynamic range of sound. The person needs to have the amplitude of sound which is delivered by a circuit aiding hearing to be compressed. This compression is required so that normal sounds, including the normal sounds included in human speech, do not exceed the limited-amplitude sensitivity range and/or the comfort range of the listener. In an artificial hearing device employing implanted electrodes special care must be taken to keep the amplitude of signals, corresponding to sound sensed by the user from the implanted electrodes, within the user's comfort zone.
One approach to implementing a gain control circuit for control of sound amplification is shown in U.S. Pat. Nos. 4,422,049 and 4,521,738 to Akagiri et al. Akagiri et al. describe a gain control circuit for controlling amplification which exhibits a very rapid attack time and which is sensitive to high and low signal levels alike. To manipulate such attack time, Akagiri et al. employ, a dual-acting AGC circuit which uses a feedback system and dual charging capacitors for the purpose of attaining a relatively long release, or recovery, time for small signals without adversely affecting the attack time for large signals.
Another approach to control of gain is shown in U.S. Pat. No. 4,371,842 to Lee. Lee is directed to controlling the gain of amplification of a pulse, or burst, signal as opposed to amplifying continuous analog signals. Lee uses feedback to control the gain of an input amplifier. A dual action is provided by Lee's gain control circuit through a time-out mechanism which switches the AGC attack time to fast if no signal has been detected for a predetermined length of time. If signal pulses are present then the circuit gain stays constant and the amount of suppression of the input signal is dependent upon the peak amplitude of the signal pulses.
U.S. Pat. No. 4,531,229 to Coulter describes a dual acting feedback AGC circuit affecting attack time so that an increased signal level causes a faster attack time. Two capacitors are charged by different paths in order to obtain charging of a quickly charging capacitor and of a slower charging capacitor. The voltage on the capacitors discharges through the same path at the same rate for a single release rate.
U.S. Pat. No. 3,665,332 to Campbell shows a single capacitor within an AGC circuit. The capacitor is discharged more quickly in response to larger signals than smaller signals, thereby according minimal "shadowing" in binaural aids for the hearing impaired. The circuit of Campbell accomplishes this selective discharge through a continuously variable resistance in the discharge path of the capacitor. The feedback voltage controls the value of this variable resistance. The feedback voltage is held on a capacitor. The capacitor thereby establishes a delay time for a change in discharge resistance. The larger a signal causing a reduction in the discharge resistance, the longer the time after the disappearance of the large signal before the resistance returns to the level appropriate to following softer sounds. Alteration of the delay time of the circuit of Campbell appears to be limited to manually changing the value of the capacitor or of other components within the circuit.
U.S. Pat. No. 3,920,931 to Yanick describes a feedback circuit controlling attenuation of an input signal at two suppression gain ratios. A primary feedback path, employed when the signal is soft, controls an input shunt resistance. If either the average level, or a band-passed peak level, signal intensity goes above a predetermined threshhold, then such shunt resistance is immediately lowered.
U.S. Pat. No. 4,202,238 to Moog describes a dual channel system receiving two input signals. The strength resultant from feeding back one signal is used to reduce that signal and to simultaneously boost the other signal.
U.S. Pat. No. 4,398,261 to Saint-Oyant et al describes the control of gain by digitally switching different attenuators into the signal path. U.S. Pat. No. 4,459,557 to McQuilken shows basic feedback for compression limited amplification of audio signals. U.S. Pat. No. 4,512,350 to Cimilluca describes a circuit which does not alter gain with reference to different input or output signal levels, but rather switches gain dependent upon elapsed time from a reference time.
All the patents discussed in the preceding paragraphs show circuits generally employing a feedback signal. In a feedback system, the system signal output is detected and then the amplification gain is controlled proportionately to this detection. In other words, the output signal sets the gain of the amplification. A disadvantage of feedback is the speed at which one can control the output level. The input signal is required to reach the output before the output level is established in and by a feedback system. This hampers achievement of a very fast attack time for the AGC control amplifier and also results in overshoot. The maintenance of a gain level, and the release, or recovery, from this maintained gain level may also have deleterious effect on the attack time. A fast attack time is always desirable for signals of all levels.
Such prior approaches show, however, in aggregate, that considerable design effort is devoted to the flexible automated control of gain during amplification which is appropriate for many purposes, particularly during amplification of sound in aid to the hearing-impaired.